Out of curiosity – which type do you think the person in question is?
There is some truth to socionics, I've noticed. I used to think it's not that important, after all I was in a good, long-term relationship with an LII when I was first introduced to socionics.
So I thought, fuck that, as long as we love each other and get along, that's all that matters, right? Yet the reason I decided it's not going to work out in the long run is ridiculously easily explained by socionics – our motivations and ambitions didn't meet. He's an alpha introvert Sp, and is fine with living a quiet, comfortable life with simple pleasures and light-hearted fun. And I'm not. I realized I couldn't afford being emotionally invested in someone with very few goals in life, and especially someone who considers my ambitions megalomanic and not only refrains from pushing me towards my goals but also bluntly refuses to help me achieve them.
And I could easily imagine several similar scenarios. For instance, conflicts between Fe/Fi valuers could really drain you in the long run. And yeah, two infantiles might still expect – unconsciously if nothing else – caregiver behavior from each other and end up feeling that their needs go unfulfilled.
Idk. I'd probably stick with quadra members. Then again if you don't encounter this sorta problems, who's to say you couldn't work it out? Not all dual relationships work out either, after all. Try it. Talk to them. Express your needs and reservations. Good communication works wonders.